1. Field of the Invention
This invention relates to a fixed geometry gas turbine combustor which produces very low NO.sub.2, CO, and total hydrocarbon emissions in which the flow of combustion air and dilution air is redistributed over the entire range of operation of the gas turbine using a pneumatic valve in the form of an inspirator.
2. Description of the Prior Art
Increasing concern about acid rain and ambient air quality has accelerated the development of ultra-low emission, natural gas combustion technologies for use in boilers, furnaces, incinerators, and more recently, gas turbines. This concern is focused not only on the oxides of sulfur and nitrogen, but also on carbon monoxide and total reactive hydrocarbons.
Of all the pollutants resulting from natural gas combustion, experience has shown that NO.sub.x has been the most difficult to minimize from a practical standpoint. Various approaches have been developed for reducing NO.sub.x emissions. However, the resulting reduction is insufficient in many cases to satisfy stringent air quality standards. Common modifications to natural gas combustion processes reduce NO.sub.x emissions at the expense of reduced equipment efficiency and very often at the expense of increased carbon monoxide (CO) and total reactive hydrocarbon emissions.
The basic approaches for lowering NO.sub.x emissions focus on reducing the concentration of free oxygen, residence time, and combustion temperature in the combustion zone. Various proven practical combustion technologies for reducing NO.sub.x formation include injection of diluents into the combustion zone, such as excess air, steam and water, homogeneous combustion, staged firing, recirculation of combustion products and flue gases, and heat removal from the flame. However, the only practical approach that has reduced NO.sub.x emissions to single digit levels is premixed combustion. Accordingly, an advanced dry combustor to achieve ultra-low emissions for all combustion pollutants, that is, NO.sub.x, carbon monoxide, and total reactive hydrocarbons, would apply the following techniques: Fuel/air premixing; high excess air combustion; and intensive turbulence, mixing and combustion products recirculation.
When such a combustor is applied to a gas turbine, several problems arise due to the specific environment required by a gas turbine combustor. Among the problem areas are: performance of the combustor when operating at high turndown over the entire range when the gas turbine operation changes from full load to idle; capability of the combustor to meet the requirements for gas turbine combustor applications, such as combustion intensity and pressure drop across the combustor; and maintaining the required combustor wall temperature over the entire operating range.
A one-shaft gas turbine is operated in such a way that total air flow is maintained constant over the entire range of turbine operation while the fuel flow rate drops from 100% at full load to about 25% at idle. This corresponds to a change in the stoichiometric amount of air required for complete combustion of the amount of fuel present from about 2.5 at full load to about 9.5 at idle.
Based on overall performance required by a gas turbine combustor, including operating range, combustion intensity, pressure drop and turndown control, certain combustor geometry options are available which produce ultra-low emissions. One such option is a fixed geometry combustor, the major advantage of which is that the gas turbine will not have any moving parts inside the pressurized machine. This is very important from the point of view of the overall turbine reliability and availability. U.S. Pat. No. 5,121,597 teaches a fixed geometry gas turbine combustor having a combustion sleeve, a combustion sub-chamber disposed at an upstream end of the combustion sleeve with an air and fuel supply system, and a main combustion chamber disposed downstream of the sub-chamber and having an air and fuel supply system, and formed in such a manner that the start up of the gas turbine is effected by the hot combustion gas generated in the sub-chamber. A fuel nozzle is provided in the sub-chamber for injecting the fuel during a change in the gas turbine rotational speed. As a result, the flow rate of fuel supplied to the nozzle for combustion in the sub-chamber, even during startup acceleration of the gas turbine, does not increase.
U.S. Pat. No. 5,054,280 teaches a fixed geometry gas turbine combustor having an auxiliary burner provided in the interior of a first-stage combustion chamber located upstream of the combustor, the auxiliary burner being fired to hold the flame formed in the first-stage combustion chamber and being extinguished to cause the first-stage combustion chamber to serve as a premixing chamber. When the auxiliary burner is fired, a diffusion-combustion flame and premixed flame are formed in the first-stage combustion chamber and second-stage combustion chamber, respectively. When the auxiliary burner is extinguished, the premixture formed in the first-stage combustion/premixing chamber together with the second-stage premixed combustion flame maintains the flame within the second-stage combustion chamber, whereby the first-stage fuel also undergoes premixed combustion. In this manner, fuel introduced into the first and second stages undergoes complete premixed combustion.
U.S. Pat. No. 4,292,801 teaches a fixed geometry dual stage-dual mode combustor for a combustion turbine having a first and second combustion chamber interconnected by a throat region in which fuel and air are introduced into the first combustion chamber for premixing therein. Additional fuel and air are introduced near the downstream end of the first combustion chamber and additional air is introduced in the throat region for combustion in the second combustion chamber.
U.S. Pat. No. 4,773,846 teaches the use of an ultrasonic fog generator for injecting a fog into the air introduced into a combustion chamber in order to improve the efficiency of the combustion chamber and/or reduce the noxious emissions in the exhaust of the combustion chamber. Means for controlling the fog generator include a pneumatic control system responsive to a control signal for controlling the supply of compressed air and the supply of water to the fogging device.
A second geometry option for a gas turbine combustor is a variable geometry combustor. See, for example, U.S. Pat. No. 4,766,721 which teaches a two-stage variable geometry combustor for a gas turbine in which fuel is supplied from primary fuel nozzles for combustion in a primary combustion chamber under low load conditions and premixed fuel/air is supplied to a secondary combustion chamber downstream of the primary combustion chamber, enabling combustion in both the primary and secondary combustion chambers under high load operating conditions. An air-bleed passageway is provided on the downstream side of air openings for intake of secondary combustion air to be mixed with the secondary fuel and communicates with the outside of the combustor. The air-bleed passageway is provided with a regulating valve whereby secondary combustion air, under low load operating conditions, is bled from the secondary combustion air supply so that the fuel/air ratio of the secondary premixed mixture does not become excessively lean under low load operating conditions. U.S. Pat. No. 5,125,227 teaches a variable geometry combustion system for a gas turbine having a fuel nozzle displaceable within a venturi section of the gas turbine combustor, thereby altering the gap in the venturi section to vary performance and stability in the combustor and reduce NO.sub.x emissions. A variable geometry combustor for gas turbines is also disclosed by U.S. Pat. No. 4,255,927 which teaches a combustion system for a gas turbine in which excess air is injected into the reaction zone of the combustor to produce a desired air/fuel mixture which lowers combustion temperature and, thus, NO.sub.x emissions. A control mechanism consisting of a valve control unit is disposed external to the turbine and is used to control combustion efficiency over a wide range of turbine load by directing air flow in an inverse relationship from a compressor between the reaction zone and the dilution zone of the combustor. A variable geometry approach is also taught by U.S. Pat. No. 4,597,264 which teaches a device for regulating the supercharging of an engine in which a pneumatically controlled pressure limiting valve is disposed in a branch of the supply duct of a turbine of a turbo compressor unit and operated to regulate the flow rate of gas supplied to the turbine.
The main disadvantage of variable geometry combustors is the need to have a valve inside the machine. As a result, any valve repair or replacement would require a shutdown of the turbine.
Another approach for reducing NO.sub.x emissions is taught by U.S. Pat. No. 4,110,973 which teaches a gas turbine engine power plant having a water injection system consisting of a water/fuel mixing device for injecting water into the engine as it is conducted to the engine combustion chambers.
None of the fixed geometry gas turbine combustors known to us are able to operate with ultra-low NO.sub.x emissions over the entire range of turbine operation.